1. Field of the Invention
The invention relates to a semiconductor device, in particular, a semiconductor device including an ESD protection element having a high ESD protection characteristic.
2. Description of the Related Art
Conventionally, for addressing ESD, various types of semiconductor devices having protection circuits for the semiconductor devices are proposed. For example, typically as shown in FIG. 6, an internal circuit 56 is protected by connecting a PN junction diode 52 between an input output terminal 50 and a power supply line 51, connecting a PN junction diode 54 between the input output terminal 50 and a ground line 53, and connecting a PN junction diode 55 between the power supply line 51 and the ground line 53. The ESD is an abbreviation of Electro-Static Discharge and means the discharge of static electricity.
However, as the miniaturization of elements is enhanced for a demand for higher speed and so on, the electrostatic breakdown tolerance of a semiconductor device is decreased and thus a more proper ESD protection element is essential. Japanese Patent Application Publication No. 2006-128293 discloses a BiCMOS type integrated circuit including a MOS type transistor as a high breakdown voltage element and an NPN bipolar transistor as a low breakdown voltage element, which uses the low breakdown voltage NPN transistor as the ESD protection element, its problem, and a means of solving the problem.
Furthermore, Japanese Patent Application Publication No. Hei 05-90481 discloses using an NPN bipolar transistor between a power supply line and a ground line as an ESD protection element instead of using a PN junction diode, in which the base and emitter are connected by a resistor. Japanese Patent Application Publication No. Hei 06-177328 discloses using a MOS type transistor as an ESD protection element of which the snapback voltage is decreased so as to enhance the ESD protection characteristic.
Although details will be described below, the snapback voltage means a trigger voltage to start discharging static electricity to a ground line when a surge voltage by large static electricity is applied to an input output terminal or the like. When the snapback voltage of a protection element is lower than the snapback voltage of an element to be protected, static electricity is discharged to the ground line through the protection element, and the element to be protected is protected from the static electricity.
Japanese Patent Application Publication No. Hei 05-90481 discloses an additional ESD protection element 59 as shown in FIG. 7 in the same structure as the structure shown in FIG. 6 for a case in which static electricity is applied between a highest potential terminal as a power supply line 51 and a lowest potential terminal as a ground line 53. Conventionally, a parasitic PN junction diode 55a shown by a dotted line which uses the N type epitaxial layer as the cathode and uses the P type semiconductor substrate as the anode becomes a discharge path of static electricity applied between both the terminals and protects an internal circuit 56. The parasitic PN junction diode 55a is a replacement of the PN junction diode 55 in FIG. 6.
However, since the enhancement of miniaturization and so on cause the increase of the impedance of the discharge path of static electricity and so on, the parasitic PN junction diode 55a does not effectively work and a discharge path of static electricity occurs through some junction in the internal circuit 56, thereby causing a problem of breaking the junction in the internal circuit 56. Therefore, for addressing static electricity, the additional ESD protection element 59 is provided in which an NPN bipolar transistor 57 of which the base and emitter are shunted by a resistor 58 is connected in parallel with the parasitic PN junction diode 55a. 
When a positive voltage by static electricity is applied from the power supply line 51 to the collector of the NPN bipolar transistor 57 connected to the power supply line 51 and a negative voltage is applied from the ground line 53 to the emitter connected to the ground line 53, the NPN bipolar transistor 57 breaks down at the time when the voltage reaches a collector-emitter breakdown voltage BVCER or more in the state where the resistor 58 is connected between the base and emitter of the NPN bipolar transistor 57.
On the contrary, when static electricity is applied as a negative voltage to the power supply line 51 and as a positive voltage to the ground line 53, the base-collector junction is forward-biased through the resistor 58 and clamped.
Therefore, the internal circuit 56 between the power supply line 51 and the ground line 53 is protected from static electricity by the additional ESD protection element 59 which is formed by the NPN bipolar transistor 57 and the resistor 58, is connected in parallel with the conventional parasitic ESD protection PN junction diode 55a, and has a lower breakdown voltage. A portion of the discharge path of static electricity lies inside the semiconductor substrate, and other portion lies on the surface of the semiconductor substrate.
However, a semiconductor device used in a mobile phone or the like which requires a lower voltage operation needs other ESD protection element which breaks down by a still lower voltage and forms a static electricity discharge path such that a more portion of the path lies inside the semiconductor substrate for heat radiation when static electricity is applied.